Which? have published advice on how best to buy double glazing windows. Photograph: Anglian picture

Double glazing salespeople are using misleading claims and hard-sell tactics to get homeowners to sign on the dotted line, according to an investigation by Which?.

The consumer organisation used undercover researchers to test the £1.2bn-a-year double glazing industry, inviting four major companies – Anglian Windows, Everest, Weatherseal Home Improvements/Zenith Staybrite and Safestyle (UK) – to provide quotes for double glazing at four addresses around the UK in March.

A senior trading standards officer and sales expert then listened to recordings of the visits and analysed the tactics used. These included claiming to be a part of a non-existent government scrappage system.

Which? also found:

• Six out of 18 salespeople made misleading claims, such as: "We've been tested by the government and were chosen for its scrappage scheme" (an Anglian salesperson) and: "By law, through trading standards, you can only give up to a 10-year guarantee" (a Safestyle UK salesperson);

• Seven out of the 18 salespeople offered a discount if the undercover researcher signed up immediately;

• Fifteen out of 18 offered significant savings, often half the "original" price – the biggest drop was £16,926 from an Everest salesperson;

• Many of the salespeople offered a lower price with a "credit deal" but working out how much you'd pay was incredibly difficult.

• One Zenith salesperson asked the researcher to sign their name twice to confirm he'd told her the relevant information – a psychological technique to warm up customers.

Peter Vicary-Smith, chief executive of Which?, said: "As a homeowner, replacing your windows is one of the most expensive jobs you're likely to face, which is why we're so disappointed to discover salespeople from the biggest names in the business making outlandish claims and using hard-sell tactics."

Which? said it hoped the introduction of the Double Glazing and Conservatory Ombudsman Scheme, launched in May, would help increase transparency of prices.

In response to the report, Everest Limited said: "We never deprive customers of the opportunity to make an informed decision. We are extremely disappointed by these findings and reject accusations of breaking regulations. Our offers are genuine. We offer a price promise, and a seven-day unconditional cancellation period on all sales. 'False claims' go against our training, guidelines and documentation."

Anglian Homes said: "All Anglian sales representatives are comprehensively trained but we do recognise that very occasionally mistakes are made and in this instance it appears that there has been some confusion surrounding the Anglian window scrappage scheme."

It said it would "once again make it clear to all our sales representatives that the two are unrelated and there is no government-backed scrappage scheme".